Register to reply

Race car suspension Class

by Ranger Mike
Tags: class, race, suspension
Share this thread:
Ranger Mike
Jul30-09, 02:53 AM
Sci Advisor
PF Gold
P: 1,473
Ok, here is how we determine the proper springs for each corner of the race car. you asked about loads on each wheel..well here is an example of our old door slammer running on a medium banked asphalt track.
Stock suspension with solid rear axle.
We calculated that it is under 1.3 Gs in the turn. " F= ( m*v^2 ) / R " is correct formula
one more piece to ponder..

from our cone killing days in SCCA Autocross..skid pad testing ,,go to parking lot, airport,,what ever, set up circle 200 to 300 feet in diameter, drive around the cirle as fast as you can without spinning out..
G = 1.225 x R / T squared
R= Radius of the turn in feet
T = Time in seconds to complete a 360 degree turn

typical Corvette corners at .84gs
road race sedan like Tran Am 1.15 Gs

if you know the tire performance curve from the manufacturer charts weight (vertical load in static pound) vs Traction (lateral load in lbs) you can calculate the Cornering efficiency.

This particular car weighs 2800 lbs. of 35% of weight will transfer under 1.3 G
and 75% will be on front end due to engine weight and corner loading

2800 lbs. X .35% = 980 lbs. transferring or loading tires

75% of 980 lbs. = 720 front end weight
divided by three to determine wheel rate ( two front springs and sway bar )
so we need wheel rate of 240

Wheel rate = (Length of A-arm divided into inside frame mount point to center of spring mounting point) squared

times spring rate

now the hard part
get out the tape measure and measure bottom front A-arm length
1. inside frame mount point to center of outside ball joint
2. distance from inside frame mount point to center of spring mounting point

stock Chevy A-arm is
16.5 inch inside frame mount point to BJ and 9 inch from inside frame mount point to center of spring pocket
assume you have a 800 lbs. spring
wheel rate = 9 / 16.5 = .54

.54 x .54 x 800 = 233 lbs. spring required to handle weight transferred

run a little stiffer sway bar and tune from here..
Chances are the Gs are off a little but we need a baseline t ostart andthis is as good as any.
Ranger Mike
Jul31-09, 05:10 AM
Sci Advisor
PF Gold
P: 1,473
We can see the effect of wheel rate regarding spring placement. Conventional coil over shock A-arm layout and the Formula Car inboard suspension which has a lot less unsprung weight.
Attached Thumbnails
whl rate con sus.jpg   whl rate  inbd sus 001.jpg  
Ranger Mike
Aug1-09, 10:28 AM
Sci Advisor
PF Gold
P: 1,473
gotta go wrench on the race car..up next is differnet types of control arm suspension
let me know ifin i missed something and will try to cover it?
Aug4-09, 10:46 AM
P: 10
Great information! I wish I was at the point in my automotive tinkering to design a suspension. Unfortunately I'm at the will of aftermarket parts manufacturers until then! Interesting to read, though.
Ranger Mike
Aug11-09, 05:56 AM
Sci Advisor
PF Gold
P: 1,473
Lets look at the true suspension mounting points on a Formula Car. Note that once you measure the FC, it is pretty easy to determine the Camber change regarding Bump and Droop. Remember..its all about tire contact patch.
Attached Thumbnails
F1 il.jpg   bump droop.jpg  
Ranger Mike
Aug11-09, 06:02 AM
Sci Advisor
PF Gold
P: 1,473
Lets look at the suspension during corner entry regarding chassis Roll. You have to control Sprung Weight and its effect on weight transfer, i.e. tire loading...note the change in Roll Center location..camber changes. You have to use the proper springs and Anti Roll Bar ( sway bar in Kentucky) to counter the Roll.

Question - How can we re-design the chassis and make it better at handling the effect of chassis Roll??
Attached Thumbnails
roll.jpg   effects of roll.jpg  
Sep4-09, 11:46 AM
P: 21
Mike.............very interesting reading.
Question..........Im running a winged pavement sprint car. Something that has always troubled me is the height of the front and rear roll centers on a typical sprint car with front and rear panhard bar. Front roll center height is usually around 11" and level side to side and approx 3" to the right.
Rear panhard height is usually around 13-14" and runs uphill 1 - 1 1/2" and approx 3" to the right as well.
Tire temps seem not bad.........with all tires around 115-120 across the tire with the exception of the right rear which usually runs about 140.
I am wondering what effect lowering the roll centers both front and rear would have ?

Also..........the more static caster I run.....lets say 10deg on the right front and 7deg on the left front ( this is pretty typical of a pavement sprint car )...........if I increase this, does this jack more or less weight into the car as I turn to the left ?? In other words.....if I increase the amount of front caster will this free the car up in the center of the turn ???

Thanks Mike.............I look forward to your response.............Kenny
Ranger Mike
Sep7-09, 05:04 AM
Sci Advisor
PF Gold
P: 1,473
Kenny ..sorry for the delay...I just returned from Mid the checker and got a third after the fuel cell foam deteriorated and clogged the fuel filter..ugg
please let me research the answer
1. do you run a front straight axel
2. do you run a solid or in dependant rear axle?
3. do you know how much akermann you have on the front?
4. do you know how much bump steer?

i got to brush up on areo thing regarding sprints
i helped an asphault sprint guy who converted to vintage sprint and the springs were way off after he removed the to dig my notes out
will reply asap
Sep7-09, 05:48 PM
P: 21
Mike............thanks for the reply.
Sorry to hear about the foam ordeal...........been there myself.
ok.......Im running a solid front axle and a live rear independant suspension allowed.
I currently have no ackerman in the front end ( hmmm.......I just assumed this since both my steering arms have the same CL-CL where the tie rod attaches)
No bump steer as per the usual with the exception of bump input into the drag link. I would think that this would be minimal since I only have an inch of suspension travel and a 49" long drag link that I run level to start with. 9" CL-CL on the pitman arm
Im running a 375 spring on the LF and a 400 spring on the RF........these are monted on the solid axle about 7" from the king pin CL
In the rear Im running a 250-275 RR spring mounted on the birdcage and a 225 - 250 LR spring mounted on the birdcage. I would assume that there is no motion ratio involved since the birdcage goes up and down with wheel travel the same amount. I allways run 25lbs of split across the rear with the bigger spring on the RR. Left side weight is about 56.5 percent.....rear weight is about 60 percent

Thanks again.....kenny
Ranger Mike
Sep8-09, 04:51 AM
Sci Advisor
PF Gold
P: 1,473
My reference is " Circle Track Suspension by Forbes Aird published by Motorbooks International Power Pro series

Beam axles were the first type of axle used on racer cars. these were dropped in favor of the independent suspension because of the room required for vertical movement, excessive unsprung weight ( leaf spring) and forces that interfered with steering. Today's spring car beam axle has been refined to greatly reduce unsprung weight thur coil over shocks , light wheels etc...the steering problem remains. For all their problems the one piece of good news is the camber remains unchanged no matter what the chassis does ( as long as the track remains dead smooth). When you drive over a dip or BUMP on the track you encounter shimmy. A bump applied to one wheel on a beam front end would cause the wheel to steer abruptly a few degrees, because of gyroscopic effect, that steering action would be communicated through the tie rod, to the wheel on the other side which was being tilted through the same camber angle at the same time. Under certain conditions, the two wheels together would generate a gyroscopic torque that would pick up the " down side" wheel and slam the first wheel back onto the track surface toed-in. The whole cycle would continue into uncontrollable flapping of the front wheels.
Bump steer with a beam axle is tuff to cure because no single point on the chassis, for either wheel, is a fixed center of movement for both bump travel land chassis roll while cornering. In Bump, each wheel arcs around the contact point of its mate on the other side. In roll, the center of motion is near the middle of the car. So it is a given we gotta live with the Bump steer we have.

I assume the Center of Gravity (CG) front (usually the cam shaft height) and rear is Above the Roll Center (RC) front and rear. Now we know that the hot set up is soft springs to minimize chassis stress and track surface irregularities but they lead to chassis ROLL. Why not raise the RC to reduce this chassis roll ( shorten the distance between the CG and RC means a shorter lever ). ???
On a beam axle set up , when a wheel bumps ,the axle tilts. If the RC is located some distance above the ground, ( all sprint cars are) it will be forced to move sideways as the axle moves in an arc around the far side tire. This lateral shove to the car may cause the tire to break traction.
Leaving the RC near ground level would require very stiff springs ( you have a long lever from CG to RC) to control roll. This may l;ead to heavy springs up front and lighter springs on the rear thus causing " porpoising " over way..
so we have the classic compromise.
All my research says the typical spring car has a front RC between 8 and 10 inches and there rear is near axle level.
I think your RC is a little high in front and I would experiment on lowering it.
I would look into a J-Bar to replace the rear Panhard bar..Will lower the rear as well. They even make a HALO Bar.

Caster looks good and would not change it,,
RR tire temp is a little high but says you got grip driving off the turns,,maybe get more heat in the lft rear??? are you carrying the left front wheel coming off the turns?

that RR tire temp is most likely from your Toe since you wild eyed sprint car type do a lot of steering with the right foot...going like He-- until you see God then turn left!!!

all and all..sounds like a VERY Close to optimum set up...
Sep8-09, 01:13 PM
P: 21
thanks for your reply Mike.

Ok.......crank hight is 9" cam looks to be about 5.5 " above this, so ya ......14.5" CG and ya...I'll go with about a 10" front roll center. Front tires are 24" diameter and the axle is 2.25 diameter with the panhard spud mounted to the bottom of the axle.
The problem Im experiencing is the car allways seams to be tight coming off. and infact did try and carry the LF, if not both front tires. Ive got the top wing almost over the front axle to try and keep the front end down
To correct this problem I went up to a 375RR spring and a 350LR spring to try and keep the front of the car down. Im running a short 4 link ( 27.5 CL-CL ) in the rear, and since Ive put this in the car hooks up REEL hard and wants to lift the front end. The bigger rear spring definitaly helped to keep the front end down, but I feel that the car wents to step out.....not loose...........but more like the rear tires are sliding laterally across the track ( if that makes sense to you )

Sep8-09, 01:14 PM
P: 21
oh.........forgot...........does more or less caster affect weight jacking as you turn the wheel ?

Sep8-09, 05:09 PM
P: 1
Very well done, Ranger Mike. I am just about to enbark on designing and fabing front uprights for a dwarf car to lower the amount of scrub. I have some pictures of an IRL car and their uprights to start from.

My question is the effect of the height of the spindle from the lower ball joint on the front end geometry. I would like to raise it in order to lower the ride height of the car. I have not seen any articles on it (I have the Steve Smith book on race car suspensions) I have a pretty good physics background from GMI. I was going to go and do some 4-bar linkage diagrams on my own, but if you have some information, I would appreciate it.

As for the push off in the sprint car, I would say it is either in your stagger (you did not mention how much you run) or the rear steer. The car is standing up on the rear tires and the front end geometry has little to do in the equation when a is as positive as with you or a top fuel dragster!!!.
Sep8-09, 08:34 PM
P: 21
I try and run around 4" of stagger................but really......Im stuck with what ever I get from the tire truck.......which is usually between 3.5 and 4 "

LR radius rods-
lower @ 1 degree down
upper @ 1" spread at other words if the bird cage spacing was 5" CL-CL then the radius rods would be 6" CL-CL at the chassis ( I have spuds to run them both down hill 1 degree, but have found that the car hooks up SIGNIFIGANTLY better with the upper radius rod uphill )

RR radius rods-
lower @ 1 degree up
upper @ 1" spread same as LR

I would really like to try and make the rear 4 link work with sum other changes..............I feel Im very close, but would love to hear any input.
Along with the rear spring rate increase I went to a straight 6 valve RR shock, wich also helped.
I would like to run as little as 2" of stagger............I feel if I can get the car to turn true the center of the corner with this, that the car should come off the corner MUCH harder...yes indeed....much like a top fuel dragster.

Thanks.....kenny the general rule for rear panhard bar angle 10% of total length ??? In other words if your panhard bar was lets say 25 " then you would want to be no more then 2.5" higher on the frame ???
Ranger Mike
Sep9-09, 03:58 AM
Sci Advisor
PF Gold
P: 1,473

When a car with positive caster turns left, the left front corner will rise and the rt. ft corner will dip. The amount of these changes depends on the amount of pos. caster used combined with the spindles steering axis inclination angle. The steering axis inclination angle multiplies the effect of the pos. caster and associated corner lift and drop. The greater the steering axis inclination, the more posative caster will change the corner height of the car as the wheel is steered. This effect is caused by the curved path that the spindle pin follows as it is turned about the steering axis.
As the car is steered left and the left ft corner rises, the result is the same as jacking weight into that corner. The chassis gains weight at the left front and right rear corner, and loses weight at the rt. ft and left rear. This effect takes some cross weight out of the chassis. The more positive caster used at the left frt. and the greater there steering axis inclination angle, the greater the loss of cross weight in the chassis as it turns left.
Kenny, whe nyo uscale the car you can see this as you crank the steering wheel.
btw..what is the king pin angle...and steering axis inclination angle you are running??
Ranger Mike
Sep9-09, 04:18 AM
Sci Advisor
PF Gold
P: 1,473
i am working on it..also kennys question
Ranger Mike
Sep9-09, 04:41 AM
Sci Advisor
PF Gold
P: 1,473
jaybee 17
Overall height of the spindle upright effects Roll Center (RC). shorter spindle height ( upper Ball Joint to Spindle shaft center line) with everything else the same, will LOWER the RC height. This is because the shorter spindle produces a longer instant center. the shorter instant center to the wheel it is drawn from , the higher the Roll Center.

you can change the RC by shortening the lower BJ center distance relative to the Spindle shaft center line as well.
you can lower the RC by raising ( relative to the track surface) the inboard upper A-arm mounting point.
you can lower the RC by lowering ( relative to the track surface) the inboard lower A-arm mounting point.
get performance trends software to take the headache out of the calculations..

I would look at three link rear suspension to cut my teeth on,,,,the four link is very tricky to learn...
Ranger Mike
Sep9-09, 05:27 AM
Sci Advisor
PF Gold
P: 1,473

Panhard bar should be mounted level at the desired RC height to begin with... don't know about that rule of thumb.
minimum stagger you need is:
D + .5 (TW) x CL \ D - .5 (TW)
where D is the track diameter in feet
TW = rear track width of the car in feet ( divide inch measurement by 12)
CL = the left rear tire circumference

example- race track has 200 ft. diameter
car has 64 inch track (64 / 12 = 5.33 inch)
left rear is 86 inch circumference.
run the numbers and you get 88.33 inch right rear circumference or MINIMUM 2.33 inch stagger.
if you run a spool add 10 percent to minimum stagger..if the track banking is 0 to 10 degrees, add another ten percent..if banking is 15 degrees up..take off ten percent from min. stagger

Four link has advantage of good forward bite because the upper links are mounted up hill. anytime the forward link is up hill the the rear end is trying to go up under the chassis during acceleration and this increases loading on the tires, these 4 links setups create oversteer as the body rolls. the right rear is pushed back and the left rear is pulled forward which helps the car turn going through the corner without breaking traction.

all I can tell you is ball park settings on the links..uppers should be 15 to 17 degrees up hill angle. the right upper max angle is 18 degrees and min. angle is 14 degrees.
the left upper usually has 3 to 5 degrees less angle than the right so the two links will be parallel through the corner. the lower links are usually 2 inches shorter. this reduces roll oversteer. the most common setting on the lowers is 5 degrees down.

Register to reply

Related Discussions
Car Suspension Explanation General Engineering 6
How does a Physics class differ from an Engineering class? Academic Guidance 38
In class final for a graduate EM class? Academic Guidance 3
Car Suspension Engineering Systems & Design 3
Spring suspension on car Introductory Physics Homework 4